The present invention relates to photographic printing and more particularly to an improved vacuum frame.
One of the most useful tools in the graphic arts industry is the vacuum contact frame which is used for making contact photographic prints. The vacuum frame should be distinguished from the vacuum platen or easel, which is used in cameras or enlargers, to hold large sheets of film at the focal plane of the camera lens or in focus. In contact printing, a transparent mask sheet is placed in intimate contact with a sheet of light sensitive material.
In the two major graphic arts applications, the light sensitive material may be photographic film or photo sensitive printing plates. For this discussion, we will call the photosensitive medium a film. The mask may contain a positive or negative image, depending on whether the photo sensitive material responds in a positive or negative way to light (e.g. some photo printing plates are positive responding). During the exposure, light passes through the mask and into the light sensitive film. Where the mask is opaque, the light is blocked and the underlying photo film remains unexposed. By this means the pattern of the mask is transferred to the photo film.
In order to retain sharpness during the transfer exposure it is necessary for the mask and photo film to be in intimate contact. If the mask and film sheets are merely pressed together, bubbles of air will remain between them and the resulting uncontrolled separation will cause local blurring and loss of definition of the transferred image.
Vacuum frames have been in use for many years to help prevent this problem. In the vacuum frame, the two sheets of film are placed under a glass top plate and are supported on the bottom by a flexible pressure blanket. The edges of the blanket are sealed against the glass plate in such a way that the air may be drawn out of the resulting glass/blanket chamber. This evacuation of air from the chamber causes the external air at ambient pressure to force the blanket up against the mask/film sandwich. The force of the blanket against the film helps force out the air from between the film sheets. A vacuum pump draws off this air so that, once full contact between the sheets is made, the contact will remain for the duration of the evacuation.
There are two significant problems with this arrangement. The first is that it takes substantial time to withdraw the air from the space between the film sheets after the volume defined by the blanket has been evacuated. A second problem is that under strong vacuum, the high pressure of the blanket tends of press the edges of the film sheets together so as to form a seal which traps a residuum of air between the sheets, thus creating bubbles or pockets in which air is trapped. These bubbles keep portions of the mask and photo film separated, which results in blurred images. Moreover, the bubbles are not spatially stabilized and may "wander" around during the course of a long duration exposure.
Early attempts to alleviate the problem of trapped air bubbles involved pressing inflated bladders against the back of the mask/photo material sandwich. No attempt was made, in these early versions, to evacuate the sandwich region. An example is the photographic blueprint apparatus which is the subject of U.S. Pat. No. 628,347 McDade. This device consists of a simple frame with a glass top in which a single chambered, hand inflated bladder presses against the back of the photo medium sandwich.
A patent from a slightly later time, the patent to Jarney, U.S. Pat. No. 725,969, shows a mechanically pumped bladder arrangement which also includes a roller that is designed to squeeze out residual trapped bubbles of air.
More recently, U.S. Pat. No. 3,995,995, to Topfer, teaches the use of a simple inflatable backing bladder, in conjunction with a vacuum frame, to squeeze out remaining bubbles of air and speed the evacuation process. It should be noted that the combination fails to regulate or monitor the air flow.
Recently, a vacuum frame has been marketed which uses rollers on the backside of the pressure blanket that move from one edge of the blanket to the other. The purpose of the rollers is to progressively squeeze the air from one edge of the flm sandwich to the other. This frame is manufactured by the Theimer company of Germany. When everything works right, no air bubbles are left behind in the film sandwich and with such intimate contact it is theoretically possible to obtain sharp, controllable exposures. In practice, however, this roller arrangement has two significant defects. Some bubbles of air still remain, and the combination is slow to operate, requiring substantial set up time before an exposure can be made. This is a significant drawback in, for example, a production environment where time is an important factor.
What has been needed is a new way to extract all of the air from the film sandwich so that no bubbles are left behind. Moreover, the new way should be sufficiently gentle not to impose great pressure on the glass top plate. Further, if the evacuation process can be speeded, the productive efficiency of the vacuum frame will be much improved. According to the present invention an apparatus which exhibits both improvements is provided.
The present invention replaces roller assemblies and single chamber bladders with a progressively inflated, multichambered bladder. The bladder is divided into a plurality of interconnected and inflatable chambers. In the preferred embodiment, the chambers are constructed in such a way as to resemble flat pockets which inflate into low ceilinged cavities.
Pressurized air is introduced to a single innermost chamber, thereby inflating this chamber first. The innermost chamber is surrounded by a nested arrangement of additional chambers.
As the central chamber inflates, air bleeds to the next adjacent chamber causing its inflation sometime after the inflation of the first chamber. As each chamber of the arrangement is inflated, the inflation of the next chamber begins. This arrangement applies pressure first to a small, central area, and then, progressively, to regions closer to the edges of the film sandwich. This radial, "peristaltic" pressure wave gently squeezes the air towards all the edges of the film sandwich in such a way that no air bubbles in the film sandwich are left behind.
A further advantage accrues from this arrangement. Because of the efficiency of the concentric peristaltic motion, the air can be squeezed from between the sheets of film at a much faster rate than heretofore has been possible. This means that the vacuum frame, assisted by the pressurized blanket, can come to full stabilized vacuum in a much shorter period of time, with resulting improvements in productivity.
The innermost chamber, and all the other inner chambers of the nesting arrangement are offset to one side of the exposure area. This is because the mask/film sandwich is normally held in lateral alignment by a system of pins inserted through holes punched along one edge of the mask and photo film. In order to maintain best alignment, it is important that the pin edge of the film sandwich be pressed together first by the vacuum frame, with the residual air being gently squeezed from this edge towards the far edges of the sandwich. For this reason, the nested chamber arrangement should be skewed towards the pin edge of the exposure area.
In the preferred embodiment, the air that is evacuated from the vacuum chamber is pumped through the bladder to cause its inflation. It has also been found that it is not necessary that the bladder be fully sealed. A continuous air flow is maintained by having a bleeder valve leak air continuously into the vacuum chamber, maintaining the vacuum pump on during the entire depressurization period. In this arrangement, the relative pressure of the bladder against the pressure blanket and film sandwich can be fully controlled. Under preferred operating conditions, the applied pressure differential is kept at less than one-half atmosphere. It has been found experimentally that this reduced pressure actually causes a quicker and more uniform evacuation of the air from between the sheets of the contact masking film/photo film sandwich. The bladder need not contain a large volume of air and, in fact, can be quite thin, thereby enabling a rapid sequential inflation of the chambers.
In experimental models, exposures made appear to be sharper than those made using previously existing commercial frames. Moreover, the evacution time has been found to be nearly an order of magnitude less than the time required by the best of the prior art frames. Accordingly, a substantial increase in productivity is possible through the use of contact photo frames according to the present invention.
An alternative embodiment of the invention employs bladders with a nested chamber arrangement such that the innermost chamber is at the center of the pressure blanket instead of being displaced towards one edge. The surrounding outer chambers are arranged symmetrically, centered on the inner chamber. As the bladder assembly progressively inflates, it first applies a holding pressure to the central portion of the mask/film sandwich. This arrangement is thought to have advantages, especially in pump down speed, in those systems where pin alignment is not to be employed. In yet other embodiments the nested chambers are displaced so that pressure contact begins at one corner of the mask/film sandwich.
A further embodiment uses a different method of construction of the multiple chambered bladder assembly. In this embodiment, partitions are formed so as to directly connect the bottom side of the pressure blanket to the lower wall of the bladder assembly. These partitions are arranged in such a way as to create a plurality of chambers, each of which surrounds a portion, or all, of the inner chambers. This embodiment is more obvious than the preferred one, but it is harder to construct.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.